Non-equilibrium Heat Exchange Research Articles

Non-equilibrium heat exchange and multi-coupled nature of mass transfer in solvent removal of propellant grains

In-depth knowledge of the heat and mass transfer mechanisms in removing the solvent of the propellant grains is crucial for optimizing process conditions as well as obtaining quality products. In this regard, this work developed an industrial-scale fixed bed test system used to study the solvent removal nature of the propellant grains. A transient model for predicting the non-equilibrium heat exchange and the solvent vaporization coupled with the adsorption of the water vapor was proposed as well. Results showed that the developed model could predict the hot air convection of the propellant grains, and the maximum relative deviation of the simulated results and the test data was 9.1%. In addition, the local profiles of the solvents mass fraction in the fixed bed had a non-uniformity in the early stage of the solvent removal, but it tended to be uniform as time went on. The propellant grains in the bottom region of the fixed bed have higher water content than that in other regions at the end of the solvent removal. The temperature of the propellant grains gradually increased due to the convective heat exchange, which led to the thermal non-equilibrium between the hot air and the propellant grains degraded.

Improving the Efficiency of a Gifford–Mcmahon Cryogenic Refrigerator

A numerical study of the performance of a single-stage Gifford–McMahon refrigerator was performed. A description of the mathematical model of the refrigerator operating with an unsteady flow of helium is given. Adequacy of the mathematical model was confirmed by comparing its results with published reliable data on tests of a refrigerator with cooling capacity of 30.2 W at 80 K. Using the wave approach, a method was developed that made it possible to study the time behavior of helium temperatures at direct and counter-flow helium blow at the regenerator inlet and outlet, as well as packing in its middle part. It is shown that for small amplitudes of temperature fluctuations, it is possible to reduce the length of the regenerator by 20–25% without diminishing its performance indicators. From the analysis of the operation of a single-stage cryogenic refrigerator, it is established that the main source of thermodynamic losses in it is the substantial non-equilibrium heat exchange between helium in the cold cavity and the wall of the working cylinder. A two-fold increase of heat transfer surface in the cold cavity will increase the cooling capacity of the refrigerator by 25%, i.e., up to 37.9 watts.

Active escape dynamics: The effect of persistence on barrier crossing.

We study a system of non-interacting active particles, propelled by colored noises, characterized by an activity time τ, and confined by a double-well potential. A straightforward application of this system is the problem of barrier crossing of active particles, which has been studied only in the limit of small activity. When τ is sufficiently large, equilibrium-like approximations break down in the barrier crossing region. In the model under investigation, it emerges as a sort of "negative temperature" region, and numerical simulations confirm the presence of non-convex local velocity distributions. We propose, in the limit of large τ, approximate equations for the typical trajectories which successfully predict many aspects of the numerical results. The local breakdown of detailed balance and its relation with a recent definition of non-equilibrium heat exchange is also discussed.

Numerical Analysis on the Radiation-Convection Coupled Heat Transfer in an Open-Cell Foam Filled Annulus

Forced flow and radiation-convection coupled heat transfer in an annulus filled with open-cell foam was numerically investigated at high temperatures. The Darcy-Brinkman-Forchheimer model was utilized to represent the fluid transport. The two-energy equation model was applied for the non-equilibrium heat exchange between the fluid and solid phases, while the radiation heat transfer within the foam material was solved using the P1 approximation. Two different cases of thermal boundary conditions were studied and discussed in detail, namely the inner wall with a constant heat flux while the outer wall was adiabatic (case I) and vice versa (case II). The effects of pertinent factors on the heat transfer characteristics were examined, such as the foam structural parameters and the radii ratio of the annulus. The temperature, local and average Nusselt number were predicted. The results indicate that neglecting the thermal radiation causes a large deviation in predicting the thermal performance of such foam-fluid systems. Increasing the porosity and pore diameter both promote the radiation heat transfer, while it is weakened by increasing the radii ratio. The average Nusselt number decreases as the porosity increases, while it exhibits a non-monotonic change with the pore diameter and radii ratio. Besides, case I shows a higher average Nusselt number than case II and presents an improved thermal performance.

Critical Conditions of Thermal Explosion Considering Space-Temporal Nonlocality

The equation for locally non-equilibrium heat exchange with nonlinear internal heat source was obtained by using Fourier’s law formula with the relaxation of heat flux and temperature gradient. The thermal conductivity problem for the infinite plate was studied by the numerical method at nonsymmetrical boundary conditions of the third kind. The conclusion was made that the thermal explosion delay time considerably increased as compared to the case of neglecting the space-temporal nonlocality.

Prospects of power generation from an enhanced geothermal system by water circulation through two horizontal wells: A case study in the Gonghe Basin, Qinghai Province, China

This study numerically investigates the power generation potential from a fractured geothermal reservoir at Qiabuqia geothermal area in the Gonghe Basin, Northwestern China, by water circulation through two horizontal wells. In the numerical model, the non-equilibrium heat exchange between fluid in fractures and rock matrix are considered to improve the simulation accuracy of heat and fluid transport in the subsurface. It is evaluated that in a reservoir at depths between 2700 m and 3200 m and initial temperature of 180 °C, the production temperature can maintain at 180 °C for 8.6 years and a temperature drops by 7.3% in 30 years. The production rate can maintain at 40 kg/s. The exergy analysis showed that the designed geothermal system could yield an electric power of 3.05–3.59 MW, with the flow impedance of 0.137–0.156 MPa/(kg/s) and energy efficiency of 31.0–62.4 over a 30-year period. The operation of such a system could reduce the total greenhouse gas (GHG) emissions by 0.27–0.92 Mt when compared to a fossil fuel plant. The sensitivity analysis indicates that the electric power mainly depends on fracture spacing and injection temperature, while the flow impedance and energy efficiency are controlled by fracture permeability.

Single-molecule-mediated heat current between an electronic and a bosonic bath

In molecular devices, electronic degrees of freedom are coupled to vibrational modes of the molecule, offering an opportunity to study fundamental aspects of this coupling at the nanoscale. To this end, we consider the nonequilibrium heat exchange between a conduction band and a bosonic bath mediated by a single molecule. For molecules large enough so that onsite Coulomb repulsion can be dropped, we carry out an asymptotically exact calculation of the heat current, governed by the smallness of the electron-phonon coupling, and obtain the steady-state heat current driven by a finite-temperature drop. At low temperatures, the heat current is found to have a power-law behavior with respect to the temperature difference with the power depending on the nature of the bosonic bath. At high temperatures, on the other hand, the current is linear in the temperature difference for all types of bosonic baths. The crossover between these behaviors is described. Some of the results are given a physical explanation by comparing to a perturbative master-equation calculation (the limitation of which we examine).